Helicopters currently provide the only practical means for vertical transport of payloads. So called "long line" pick up techniques have been employed but only small amounts of weight, i.e. a few hundred pounds, may be picked up by any size of aircraft. Illustrative of the long line concept are the following U.S. Pat. Nos.:
1,829,474--issued Oct. 27, 1931 to Chilowsky PA1 3,351,325--issued Nov. 7, 1967 to Cotton PA1 3,724,817--issued Apr. 3, 1973 to Simons
More in point, and the closest known prior art to the invention herein, is the Alabrune U.S. Pat. No. 2,298,912 which envisions two aircraft flying in a circle or in orbit with a payload suspended between them. That is the simple or non-critical part of the procedure. The significant part is the transaction maneuver required to enter and to exit the orbit. Successful solution of this problem according to the present invention is crucial to obtaining a practical, useful system.
Alabrune proposes to have one of his two aircraft in orbit accelerate with relation to the other so as to gain on it. The payload is thereby caused to follow a spiral path beginning at the center of the orbit circle and converging onto the orbit track. This is objectionable for several reasons.
To begin with, the required time in orbit with the payload off the ground is excessive. Payload dynamics cause it not to follow the idealized spiral path at all but to swing away from center. Also aircraft dynamics caused by crosswind or pilot induced small departures from the optimum orbit path introduce oscillations into the payload path causing it to gyrate widely and inducing large inertial loads onto the payload, the cables, and the aircraft.
In addition, it is most desirable to have the orbit and de-orbit maneuvers occur with dispatch in order to reduce the vulnerability of the aircraft to attack, where the system is used in an enemy environment, as well as to reduce pilot workload in flying this very demanding procedure. Moreover, the de-orbit maneuver should be executed in a manner to cause the payload to lift off the ground swiftly in a near vertical direction to avoid the dangers associated with a slow lift off. An example of such dangers are payload bouncing on the ground perhaps repeatedly in various places before finally being fully airborne, imposing hazards to ground personnel and to the payload.
It is desirable that the airplanes in orbit fly as slowly as possible in order to minimize the orbit radius. This is necessary to reduce the amount of cable extended and to allow the flight crew to be able to see both the payload and the other airplane in orbit to maintain proper station. At a given, reasonable bank angle and load factor, the orbit radius increases as the square of the airspeed. However, the amount of weight that the wings and the airplane can sustain (airplane plus payload) also varies directly as the square of the airspeed. Hence there exists an optimum orbital airspeed for each mission, as low as possible to minimize orbit radius, yet high enough to sustain the weight of the airplane plus payload with adequate stall margin.
The Alabrune invention requires that the aircraft fly at dissimilar speeds during the de-orbit and re-orbit maneuvers. This results in the requirement that the cargo be sustained while the aircraft are operating at non-optimum orbiting conditions, thus inherently limiting the payload to a lesser value than can be handled by the present invention.